1. Field of the Invention
This invention relates to an optical head, and more particularly to an optical head suitable for use in an optical information recording or reproducing apparatus such as a video disc, an optical memory or a magneto-optical memory for utilizing light to record information on an information carrier or reproduce the information recorded on the information carrier, and relates to a method of detecting the focus of such head.
2. Description of the Prior Art
Heretofore, optical heads have been often used in optical information recording or reproducing apparatuses and these are usually designed to condense a light beam on a moving recording medium to form a minute spot and effect the information recording or reproduction utilizing optical modulation. The optical modulation used therein depends on the presence or absence of minute concavities and convexities or apertures, the variation in reflection factor, the direction of magnetization, etc. and, in any recording method, it is necessary to follow the vertical movement of the disc and the horizontal movement of the information recording track at a high speed and at high accuracy and thereby control the position of the light spot. Also, it is desirable that the information recorded at an arbitrary position on the recording medium can be random-accessed within the shortest possible time.
FIG. 1 of the accompanying drawings shows an example in which such a conventional optical head is used in a magneto-optical memory. In FIG. 1, the light beam emitted from a laser light source 1 is made into a parallel light beam by a collimation lens 2, passes through a beam splitter 3, is condensed by an objective lens 4, passes through a substrate 5 and forms a minute spot on a magneto-optical recording medium 6. The light beam which has been reflected by the magneto-optical recording medium 6 and whose plane of vibration has been rotated by a minute angle passes through the objective lens 4 and becomes a parallel light beam and is reflected by the beam splitter 3, whereafter it is divided into two light beams by a polarizing beam splitter 7 used for differential amplification and set to a suitable angle. One of the divided light beams is condensed on a photodetector 10 by a condenser lens 8 and the other light beam is condensed on a photodetector 11 by a condenser lens 9. The light outputs detected by the respective photodetectors have the difference therebetween taken by a differential amplifier (not shown) and signal reading of a high SN ratio is effected.
The principle on which signal detection is accomplished by the polarizing beam splitter is illustrated in FIGS. 2A and 2B of the accompanying drawings. FIG. 2A shows the relation between the plane of vibration of an incident linearly-polarized light and the plane of vibration of the reflected light beam. In FIG. 2A, reference numeral 12 designates the direction of vibration of the incident light beam, reference numeral 13 denotes the direction of vibration of the light beam reflected from a portion on which information is not recorded, and reference numeral 14 designates the direction of vibration of the reflected light from a portion on which information is recorded. The information is recorded by the variation in direction of magnetization of the recording medium, and the light beam corresponds to the direction of magnetization as shown in FIG. 2A and is reflected with the plane of vibration hereof being subjected to the rotations in opposite directions by the magneto-optic Kerr effect. Usually, this angle of rotation (the angle of Kerr rotation) is a minute angle of 1.degree. or less. FIG. 2B shows the manner in which the reflected light 12 from the portion on which information is not recorded is divided into two light beams of equal intensities by the polarizing beam splitter. By the transmitting plane of vibration of the polarizing beam splitter 7 of FIG. 1 being set to the direction of 45.degree. with respect to the direction 12, the quantity of light detected by the photodetector 10 of FIG. 1 becomes a quantity of light corresponding to arrow 15 and the quantity of light detected by the photodetector 11 becomes a quantity of light corresponding to arrow 16. As a result, the difference between the two quantities of light becomes zero while, on the other hand, the two quantities of light are not equal to each other in the light beam 14 from the portion on which information is recorded and therefore, a signal output is produced to render the information-reading possible. The advantage of such differential detection lies not only in that the apparent degree of signal modulation is increased, but also in that the influences of the fluctuation of the reflection factor of the recording surface, dust, the variation in intensity of the light source, etc. are negated to a considerable degree, and the effect thereof is very great.
In the magneto-optical memory, in order that the light beam may be converged on the magneto-optical recording medium, it is necessary to detect the distance between the objective lens and the recording medium and control the optical head in accordance with that distance to ensure the light beam to be of a minimum diameter on the recording medium. For the detection of such distance between the objective lens and the recording medium, i.e., the so-called focus detection, there has heretofore been adopted a method using a cylindrical lens to detect the in-focus position by the shape of a point image or a method using a knife edge to detect the in-focus position by movement of the position of the point image.
However, the prior art optical heads, including the above-described example, are of a construction including a light source, photodetectors, etc. and therefore, bulkiness and increased weight of the head are unavoidable. Also, where the focus detecting means as described above is provided, the construction of the head becomes complex and assembly and adjustment thereof are cumbersome and further, the optical head becomes bulky and heavy. Such bulkiness and heavy weight have been great obstacles to the aforementioned high-speed and highly accurate access required of the optical head.